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VPHOT comparison stars

I am looking at my VPHOT display for a 30 second unfiltered image of CV variable LL AND taken with the Sierra Stars 24" in CA. Comparison stars go down to and include the 170 comp star. Yet when I look at the sequence for LL AND, there are three additional comp stars (176, 179 and 182) listed which are not displayed on the VPHOT image. These three comp stars are shown when plotting a VSP genereated finder chart using a DSS image and a magnitude limit of 19.0. Is there an arbitrary or an SNR based magnitude limit imposed on the comp stars that are displayed in VPHOT?

The "problem" may be in your perception. It IS recommended that you USE only comp stars with a good S/N (>100). But sometimes you can't, right? When you upload an image into VPHOT and ask it to show you the AAVSO comp stars, it will show all of them in the catalog IF they have a S/N in that particular image greater than the limit you choose in the other drop down box. So, if your VSP chart shows fainter comp stars than your image actually has in it, what else would you expect? If you save the sequence, VPHOT will remember them but the next image you upload that has too low S/Ns for some (or all) of the stars, they won't be displayed. Kinda makes sense, doesn't it?

YOU choose which stars you will use in the sequence for comps. You should choose comp stars that area about the same S/N (and color) as your target star. If your target star and all the available comp stars have low S/Ns, then you should consider taking longer exposures (and/or stacking) before attempting to measure them.

If your comp stars are well-exposed and your variable has dropped down to a really low S/N (not uncommon) your variable measurement will be plagued by the uncertainties due to its low S/N but it is still a worthwhile effort. VPHOT will report an uncertainty commiserate with that low S/N but the uncertainties in the comp star will be negligble by comparison.